Braun tube



March 5, 1935. H. G. MGLLER ET AL BRAUN TUBE 3 Sheets-Sheet 1 I venta/sMarch 5, 1935. 'H. G. MOLLER ET AL 1,993,565

' BRYVAUN TUBE Filed July 28, 1932 5 Sheets-Sheet 2 March 5, 1935'. v

HLG. MOLLER ET AL Filed July 28, 1952 a Sheets-:Shet 3 Patented Mar. 5,1935 PATENT OFFICE BRAUN TUBE Hans Georg Milller, Bergedorf, nearHamburg, and Rolf Miiller, Zehlendorf, near Berlin, Germany, assignorsto the firm of Fernseh Aktien- Gesellschaft, Zehlendorf, near Berlin,Germany Application July 28, 1932, Serial No. 625,484 In Germany July31, 1931 4 Claims.

This invention relates to Braun tubes, such as employed for purposes oftelevision, and more particularly to a method of controlling theintensity of the light.

5 When employing a Braun tube in television apparatusin'its capacity asan image-composing element the stream of electrons is deflected by meansof electrical or magnetic fields in two directions which are vertical toeach other in a manner corresponding with that in which the image isdecomposed at the transmission end by the scanning device. Beyond thisthe intensity of the spot of light on the fluorescent screen (point ofimpact of the bundle of electrons) requires to possess at eachparticular moment the same intensity as that of the corresponding imagepoint of the object scanned at the transmission end.

There are two possibilities of enabling the intensity of the spot oflight to be varied, consisting in either varying the speed of theelectrons whilst maintaining the latter constant in number, or invarying the number of electrons at a constant speed thereof. Thisvariation is performed in accordance with the image frequency amplitudestransmitted.

The former method is extremely complicated to perform, as it isnecessary, on account of the variation in the deflection at differentspeeds, to vary the deflection potentials in rhythm with the imagefrequency, which amounts to modulation of the deflection potentials withthe image frequencies.

In performing the second method of control it is necessary to vary thenumber of electrons, which are united in the spot of light, inconformity with the image frequencies.

For this purpose various proposals have already been made which,however, are accompanied by the drawback that although therate ofmovement is maintained substantially constant the expanse of the spot oflight cannot be sufiiciently confined or is not independent of thecontrol-a disadvantage, which is hardly capable of leading to exactcomposition of the image.

It is the primary object of this invention to eliminate the drawbacksreferred to, and-this is accomplished by the fact that in the Braun tube'there is provided a relatively narrow diaphragm, through which a streamof electrons passes fluctuating in intensity, and that this shutter isreproduced electrically on the fluorescent screen, i. e., the electronsare united at one point of the screen. This reproduction may occurelectro-magnetically by means of a spot-concentrating coil, 8r electro-55 statically by an electrode arrangement, for example in cylindricalfornl. The control device governing the number of electrons may alsooperate electro-magnetically or electrostatically. In order to conveythe largest possible number of the electrons emitted by the hot cathodethrough the 5 diaphragm, it is desirable to concentrate as far aspossible right from the commencement the electrons proceeding from thefilament. By variation of this concentration it is then possible forexample, to vary the number of electrons leaving the 10 diaphragm.

The concentration device maybe, for example, a Wehnelt cylinder, thepotential of which is varied in rhythm with the incoming imagefrequencies. 15 Other objects and advantages of the invention willbecome apparent as the description proceeds. The invention will now bedescribed more fully with reference to the accompanying drawings, inwhich 1 20 Fig. 1 illustrates a possible embodiment of the invention indiagrammatical form,

Figs. 2 and 3 being modifications.

Referring now to the drawings, the hot cathode 1 is surrounded by theWehnelt cylinder 2, and closely in front of the same there is providedthe electrode 3, to which is applied a slight positive potential. Theelectrons emerging from the aperture 4 are concentrated by the cylinder5, in which connection it may under certain circumstances, for the sakeof improved concentration, be desirable to fill the tube with a gas. Thefield accelerating the electrons is created by the anode 6, which islinked up with a high potential. Correct composition of the image isobtained by means of the two pairs of deflecting plates '7 and 8. Theelectrostatical deflection may naturally also be replaced by anelectro-magnetic one. The requisite current sources have not been shownin the drawings, as it is obvious that the cylinder 5 is preferablysupplied with a potential which is negative in face of the electrode 3,and that the same potential ratio exists between the filament 1 and theWehnelt cylinder 2. Under certain circumstances it might alsobe'practical to furnish the Wehnelt cylinder 2 with a potential which ispositive as compared with the filament.

In order by this method to obtain a spot of small expanse on thefluorescent screen 9, the aperture 4 will require to be relativelysmall, and accordingly the number of electrons considerably frestricted. In this respect it is perhaps more convenient to employ thearrangement illustrated in Fig. 2, in which the same method of controlis made use of, but in place of the electrostatical 2 1,998concentration provision is made for an electromagnetic one with theassistance of a so-called spot-concentration coil 10.

In the method described in the above considerable difliculties areunfortunately encountered in the practical execution, as despite the useof the spot concentration coil it is not readily possible to produce aclearly defined bright spot of light on the fluorescent screen. Bycareful investigations it was found that this is due to the fact thatexact concentration is dependent not only on the strength of themagnetic field and the speed of the electrons, but also on thedistribution of their speed.

In accordance, therefore, with the invention,- and it is only in thismanner that the method above referred to is rendered possiblethere isemployed for each definite distribution of potential, and accordingly ofthe speed, a well defined electrode and spot-concentration coilarrangement. In the most elementary form, in which the fieldaccelerating the electrons is disposed solely between the electrodes 3and 6, the potential prevails between the anode 6 and the screen 9, andthe spot-concentration coil is located in place of the anode 6, thedistances between these electrodes must be selected in the ratio of 1:2.

If any other distribution of the potential is made use of, for examplethe fluorescent screen itself placed on full anode potential and merelya part potential applied to the anode 6, an arrangement which, asdescribed later, possesses certain advantages, this spacing will requireto differ accordingly. It is only as a result of this recognition thatthe method described may be utilized at all.

In the arrangement illustrated in Fig. 2 the glass bulb is relativelylong. Since on account of the size of image to be obtained on thefluorescent screen there is a prescribed minimum distance between thescreen and the deflecting plates (if this spacing were too small thedeflection potentials would be excessively high, while moreover the lawsof proportion as regards the deflection would no longer apply), thetubebecomes relatively long owing to the, necessary ratio of 1:2.

This arrangement would then have the additional advantage that theelectrons traverse the deflection fields at a very low speed, so thatvery much smaller deflection potentials may be employed, which is aboveall of particular importance when the deflection potentials aregenerated at the transmission end, picked up at the receiving end andrequire to be amplified.

In this connection variation is desirable in the distribution of thepotential (charging of the fluorescent screen with full anodepotential). If, for example, the anode 6 is omitted entirely, the spotconcentration coil should divide the distance between the electrode 3and the fluorescent screen 9 in the ratio of 1:1.

The construction of the fluorescent screen as an electrode wouldnormally be performed by applying the material to a metal plate. In thiscase, however, it is possible toobserve the television image merelythrough the lateral, usually conical portion of the Braun tube, which onaccount of. the reflection of the glass, distortions, and limitation ofthe visual angle is extremely unpleasant. In order now to permit ofobservation against the light in the normally usual manner, thefluorescent substance, in accordance with the invention, is applied to ametal screen, which is so thin that appreciable absorption of the lighttherein does not take place.

The metallic coating may naturally not be so thin that the sameappreciably increases the resistance of the entire circuit. In theproduction of this metallic coating, which normally is applied to theinner glass wall in direct fashion, and on which there is then locatedthe fluorescent mass, difficulties may possibly be encountered insofaras the fluorescent screen charges itself.

According, therefore, to the invention, the metallic coating is providedon the side of the fluorescent screen which is directed towards thedeflecting plates, so that the metallic coating then acts in a manner ofspeaking as a Lenard window.

In the described method of concentration making use of aspot-concentration coil the cross-section of the bundle of electrons atthe point of the anode 6, or at the point of location of the deflectingplates, will be relatively large.

This occasions wider spacing of the deflecting plates in order to avoidscreening effect, and consequently a smaller deflection sensitivity. Ifnow in addition to the electro-magnetic concentration there is alsoemployed electrostatical concentration using a Wehnelt cylinder, theelectrons will leave the shutter aperture 4 at a smaller angle in space,i. e., will be bunched together in more defined form right from thecommencement, so. that the cross-section of the bundle of electrons willnow also be smaller at the point of location of the deflecting plates,which in turn results in greater deflection sensitiveness.

As a modification of the arrangement illustrated in Fig. 1 the spacingbetween the hot cathode and the electrode 3 may also be madeconsiderably larger. Upon variations in potential of the Wehneltcylinder 2 the diameter of the cathode spot then fluctuates on theelectrode 3. By means of the aperture 4 a certain section is shaded ofidependent on the concentration of the bundle of electrons. It isnaturally in this case also possible to employ as control element asocalled spot-concentration coil. Attention must naturally be paid tothe fact that the self-induction thereof is not excessive, in order toavoid failure in the case of high control frequencies.

In the arrangement according to Fig. l the electrode 3 is situated asclose as possible to the filament, and the Wehnelt cylinder 2 isapproached as far as possible towards the electrode 3. There is thenformed by the Wehnelt cylinder 2 and the electrode 3 a hollow space, inwhich is situated the filament, and which communicates by means of thenarrow aperture 4.

If new the filament or the inner walls of the electrodes discharge gas,it may occur that the pressure compensation does not take place withsufiicient rapidity through the aperture 4, so that the filament thenburns in a gas atmosphere and under these circumstances the length oflife thereof may suffer accordingly. It is therefore desirable -tofurnish the Wehnelt cylinder additionally with lateral openings, or alsoto make the same altogether of metal fabric.

In numerous cases, and more particularly if a gas filling is dispensedwith, so-called wall charges disturb exact composition of the image andthe control of the intensity. To avoid this the inner wall of the glassbulb may be furnished with a metallic coating, which is charged with asuitable constant potential, in order to render the wall chargesineflective.

For the sake of faultless operation of the tube the electrical fieldacting on the electrons should be of such disposal that the course ofthe potential is not varied independently of the deflection of thebundle of electrons. This requirement, however, is not fulfilled if theinner wall of the glass bulb is furnished with one single metalliccoating.

According now to the invention, the metallic coatings are so producedthat approximately the same potential prevails in a certain definitecrosssection. A possible form of embodiment of.the inventive idea isillustrated in Fig 3, in which like reference characters have beenemployed to denote similar parts, with the exception that in additionthere is provided a lead 11 electrically connected with the metalcoating of the fluorescent screen.

This lead 11 is also connected with a metallic coating 12, which hasbeen produced, for example, by chemical deposit, and possesses theserrated form indicated in the drawings. The intermediate spaces areengaged by the serrations of a metallic coating 13, which may beconnected, for example, with the anode 6. The zi g-zag embodiment of themetal coating represents an extremely suitable form; naturally otherarrangements are also possible, such as inter-engaging wire points. Thesame arrangement may also be used at other positions, for examplebetween the cathode arrangement and the anode 6. It is naturally alsopossible to employ a number of rings spaced slightly apart, each ringbeing charged with a somewhat higher potential than the preceding one.

It will be understood that no restriction is made to the particularforms of embodiment shown, and that various modiflcationsare quitepossible within the meaning of the above description and theannexedclaims without departing from the spirit of the invention. The examplesquoted are intended to represent possible embodiments of the basic ideaof the invention, consisting in the fact that a number of electrons passthrough a diaphragm in accordance with the oncoming image frequencies,and that this bundle of electrons is again united in electrostatical orelectro-magnetc fashion on the fluorescent screen.

What we claim as new and desire to secure by Letters Patent is: 1

1. A Braun tube including a hotcathode, a fluorescentscreen, a diaphragmadjacent said cathode for allowing the passage of electrons inaccordance with the incoming image'frequency amplitudes, and atransparent metallic coating on that side of the said screen which isdirected towards said cathode.

2. A Braun tube including a hot cathode, a diaphragm adjacent saidcathode for allowing the passage of electrons in accordance with theincoming image frequency amplitudes, an anode, a fluorescent screenadapted tohaving a potential different of that of the anode, andelectrically separated metallic coatings applied in direct fashion tothe side walls of the said tube between said anode and said screen for.avoiding variation in the potential of the field between anode andfluorescent screen traversed by the electrons. One of said coatingsextending to said fluorescent screen and adapted to have applied theretothe same potential which is applied to said screen.

3. A Braun tube including a hot cathode, a diaphragm for allowing thepassage a concentrated beam of electrons in accordance with the incomingimage frequency amplitudes, an anode, a fluorescent screen adapted tohaving applied thereto a potential difierent from that which is appliedto the anode, a transparent metallic coatingon said fluorescent screen,and metallic coatings applied in direct fashion to the side walls of thesaid tube between said anode and said screen, said coatings beingelectrically separated from each other one being connected to the anodeand one being connected to thetransparent metallic coating on saidfluorescent screen, the opposed edges of said side wall coatings havingthe shape of a serrated line.

4. A Braun tube, including ahot cathode, 9. diaphragm for allowing thepassage a concentrated beam of electrons in accordance with the incomingimage frequency amplitudes, an anode, a fluorescent screen adapted ofhaving applied thereto a potential difierent from that which is appliedto the anode, a transparent metallic coating on said fluorescent screen,and metallic coatings applied in direct fashion to the side walls of thesaid tube between said anode and said screen, said coatings beingelectrically separated from each other, one of said coatings beingconnected to the anode and the other one being connected to thetransparent metallic coating on said fluorescent screen, said coatingsincluding elongated sections confined by converging lines whichterminate at their intersecting points, the elongated sections of eachcoating extending in longitudinal direction of the tube and extendinginto the intersp'aces between the adjacentelongated sections of theother coating.

' 2 HANS GEORG MbILER.

, ROLF Mdmnn. j

